A defect inspecting apparatus according to a method using an image of an optical microscope is well known. The defect inspecting apparatus according to the method inspects whether or not a defect of a shape of an inspection object exists and whether or not a foreign object exists on the inspection object by forming an image of a surface of the inspection object which is arranged on a stage. The inspection object indicates a reticle or a photo-mask on which fine patterns of a circuit and an element are formed. Moreover, the inspection object includes a fine structural object, like a minute electromechanical component called MEMS (Micro-Electro-Mechanical System) and an electronic device such as LSI (Large-Scale Integrated circuit) which are manufactured by downscaling the fine patterns of the circuit and the element and copying the fine patterns.
In the defect inspecting apparatus according to the method, it is emphasized to obtain an even and good microscope image in an observation view. Therefore, in order to illuminate the observation view uniformly and illuminate an inspection surface of the fine structural object, a light whose optical property is uniform in all directions, that is, a light in a non-polarized light (i.e. circularly-polarized light or randomly-polarized light) is preferable.
Furthermore, as micro-fabrication of LSI, MEMS or the like proceeds and miniaturization of the reticle pattern and photo mask pattern consequently proceeds, the defect inspecting apparatus having high resolving power which can clearly resolve a shape of the pattern on the inspection surface of the fine structural objects.
Resolving power ε of the optical microscope is represented as ε=k1×λ/NA, where λ is wavelength of a light and NA is numerical aperture of an objective lens (k1 is a fixed number determined on the basis of conditions of a light source). Accordingly, in order to resolve a fine surface structure, it is required to reduce the wavelength of the light and to increase NA of the objective lens. In order to reduce the wavelength of the light, a light source with high stable output power is required, and an optical imaging system having high accuracy which hardly deteriorates in short wavelength area and which can uniformly illuminate the observation view is also required.
From a viewpoint of optics, it is not easy to illuminate the observation view with uniform light intensity. Since the observation view is illuminated simultaneously, brightness becomes very low. Therefore, exposure time of a light receiving element (multi-element type image sensor, such as Charge Coupled Device (CCD) image sensor) has to be quite long, in order to obtain high Signal to Noise ratio (S/N).
Therefore, it is technically difficult to realize the optical imaging system with high accuracy by using a short wavelength light (e.g. wavelength of about 200 nm) which is technically available at the present time. Even if the optical imaging system is realized, the defect inspecting apparatus becomes very expensive. Realizing high NA is similar to the above case. That is, it is technically difficult to realize high NA even by using a liquid immersion lens or the like which becomes notable in recent years. Even if high NA is realized, the defect inspecting apparatus becomes very expensive. High cost for the defect inspecting apparatus can be one of causes of raising cost of the fine structural objects such as the reticle, the photo-mask, LSI and MEMS. Such things above mentioned are not preferable.
Japanese Patent Application Laid-Open No. 1996-005569 (hereinafter referred to as “patent document”) discloses a scanning type particle measuring apparatus to resolve disadvantages of the defect inspecting apparatus using an imaging microscope method. In the particle measuring apparatus disclosed in the patent document, two kinds of laser beams including a p-polarized laser beam and an s-polarized laser beam vertically illuminate the same area of a wafer arranged on the stage simultaneously. A light reflected by the wafer is split into a p-polarized light component and an s-polarized light component by a polarized light beam splitter. The split p-polarized light component is converted into an electric signal by a light receiving part for the p-polarized light component. The split s-polarized light component is converted into an electric signal by a light receiving part for the s-polarized light component.
When a computer processes the two output signals sent thereto, it is detected whether or not a particle exists. Two kinds of laser beams scan the wafer with predetermined scanning width in cooperation with movement of the stage on which the wafer is arranged. Repeating the above mentioned operation, the laser beams scan the whole of the wafer surface. It is possible to carry out particle detecting over the whole of the wafer surface.
In the particle measuring apparatus disclosed in the patent document, since the laser beam scans the wafer surface, diameter of a converging spot of the laser beam is short compared with that of the defect inspection apparatus using the imaging microscope method. Therefore, the particle measuring apparatus gives very high brightness. Since problems regarding illumination intensity are resolved, high S/N ratio and high throughput are secured. Moreover, in the particle measuring apparatus, since scattered light intensity for diameter of each particle is different from each other according to polarization property, two kinds of the laser beams including the p-polarized laser beam and the s-polarized laser beam are used for measuring. As a result, particle measurement of each particle diameter can be performed with the highest sensitivity.